This inventon relates to a sideband analyzer for amplitude modulation transmitters and more particularly to the measurement of sideband amplitude response and/or sideband group delay response of such transmitters capable of broadcasting television.
The present method for measuring the performance of television transmitters is to measure the video-in to video-out group delay characteristic. A tone is modulated onto a video sinewave signal, and this signal is applied to the transmitter under test. An envelope detector detects the tone from the output of the transmitter under test, and the detected tone is applied to a phase meter. The original tone is applied through a phase shifter to the phase meter. The phase shifter adjusts the phase of the reference tone signal at some reference video frequency such as 200 KHz (kilohertz) to zero phase relative to the phase of the demodulated tone signal. This adjusts out any average phase shift through the unknown circuit, and the group delay is then measured relative to that at the reference video frequency (group delay at reference video frequency is taken at zero). A fault in this measurement is that the group delay characteristic over the lower sideband region and the upper sideband region is not measured. A composite effect of both sidebands is measured. Also, included in the measurement are the effects of the vestigial amplitude response slope of te RF demodulator, and the overall phase characteristic of the RF demodulator.